Skeletons such as those of a teenage dwarf and a girl with malformed bones were found buried alongside able-bodied dead. This indicates that human sacrifices may have been an important ritual activity among ancient hunter-gatherer tribes, according to lead study author Vincenzo Formicola of the University of Pisa in Italy.

The finds also suggest that prehistoric societies during the Upper Paleolithic period were surprisingly advanced compared to later Stone Age peoples in Europe.

Previous Clues

Formicola and colleagues' study focused on three previously described ceremonial burial sites in Russia, the Czech Republic, and Italy.

Bodies in these multiple graves appear to have been purposely selected, Formicola said. There were similarities in sex and age along with the presence of individuals with serious physical impairments.

The sites include a double burial of the so-called Sunghir children, unearthed at Vladimir, Russia, some 120 miles (193 kilometers) east of Moscow.

The bodies of a preteen boy and girl were discovered with grave goods, including about 5,000 perforated ivory beads thought to have been sown into caps and clothing.

Perforated arctic fox teeth, ivory pins, and carvings, as well as disk-shaped pendants and giant spears made of mammoth tusks were also found. The bones had been covered in red ochre, a pigment made from clay.

Since each bead would have taken more than an hour to make, the children's burial may have been planned before they died some 24,000 years ago, Formicola said.

"The enormous amount of time required to prepare all those ivory objects leads one to wonder whether this ceremony was foreseen long in advance," he said.